This invention relates to a discharge tube lighting apparatus, a light source apparatus, and a display apparatus, and more particularly to a discharge tube lighting apparatus which includes a balance coil for driving a large number of discharge tubes to emit light using a smaller number of inverter circuits and a light source apparatus and a display apparatus which use a discharge tube lighting apparatus.
In recent years, image display apparatus which make use of liquid crystal are used popularly. A display apparatus of the type described requires a light source apparatus for irradiating light from the rear side of a liquid crystal panel, and such a light source apparatus as just mentioned is usually called backlight. Together with increase in scale of a display apparatus, increase in scale also of the backlight which composes the display apparatus has proceeded, and a discharge tube is used for the backlight. However, it is not efficient to drive a plurality of discharge tubes individually by means of driving circuits provided individually for the discharge tubes. Thus, a discharge tube lighting apparatus which uses one inverter circuit to drive a plurality of discharge tubes to emit light is used. A light source apparatus that a discharge tube section including a plurality of discharge tubes is driven by such a discharge tube lighting apparatus as described above is used. Further, in a backlight of an increased scale, a plurality of inverters are used, and a backlight is formed from a number of discharge tubes greater than the number of inverters.
Where a plurality of discharge lamps are driven to emit light by a single inverter circuit in this manner, there is a problem that electric current of different magnitudes flows through the discharge tubes. To solve this problem, an apparatus which includes a balance coil has been proposed and is disclosed, for example, in Japanese Patent Laid-open No. Sho 56-86495.
Where a balance coil is used, if one of discharge tubes does not discharge, then an excessively high voltage is generated across windings of the balance coil and damages the windings of the balance coil. Therefore, a countermeasure which includes a diac disposed in parallel to each winding to protect the winding is reported. Also a system which detects an anomaly of a discharge apparatus from current flowing through a photo-coupler when a voltage generated in any one of windings exceeds a breakdown voltage of a Zener diode has been proposed and is disclosed in Japanese Patent Laid-open No. 2004-335443.
An example of a balance coil is disclosed in Japanese Patent Laid-open No. 2005-317253 and shown in FIGS. 11A and 11B. FIG. 11B shows a schematic view of the balance coil 30 in an assembled state, and FIG. 11A shows, in plan, a bobbin which composes the balance coil 30. Referring to FIGS. 11A and 11B, the bobbin 20 is shaped such that a winding frame thereof is divided into two winding regions by a main partition 22 at an intermediate portion thereof and an auxiliary partition 24 is provided in each of the winding regions so that the winding region is divided into a plurality of (two in the arrangement shown) sections. The two windings are wound separately from each other in the different winding regions on a single bobbin having the main partition 22 formed at an intermediate portion thereof, and each of the windings is wound divisionally in the two sections divided by the auxiliary partition 24. The bobbin 20 is a unitary molded article made of an electric insulating material. Thus, the two windings are provided separately from each other. A magnetic core 28 is combined with the bobbin which has terminals 26 to assemble the balance coil 30. While, in the arrangement shown in FIGS. 11A and 11B, one auxiliary partition is provided in each of the winding regions so that the balance coil 30 has a two-section structure. However, two auxiliary partitions may be provided in each of the winding regions so that the balance coil 30 may have a three-section structure. Naturally, each winding region may be divided into a greater number of sections. However, the number of sections is determined taking the number of windings, the winding wire diameter, and so forth into consideration. The magnetic core is structured such that two E type cores made of a high-permeability magnetic material such as a NiZn type ferrite material or a MnZn type ferrite material are assembled such that the leg portions thereof are disposed in an opposing relationship with each other with ends thereof held in contact with each other.
As described hereinabove, where a balance coil is used, the subject of the current balance of discharge tubes is solved. However, if a line connecting from the balance coil to one of the discharge tubes is not connected by breaking of a wiring line pattern, failure in connection of a connector, or the like, then it is supposed that a high voltage is generated in the wiring line of the balance coil which corresponds to the disconnected line. In such an instance, there is the possibility that a discharge phenomenon may occur between wiring patterns, between the windings of the balance coil, or between the wiring and the core which form a magnetic circuit, whereupon a stench is given off, the balance coil may burn out, or a printed circuit board may be carbonized.
Where the countermeasure described above against such a subject as just described that a diac is disposed in parallel to each of windings is adopted, a high cost is required for the countermeasure and merely the subject of protection of windings for a predetermined period of time can be solved. However, application of a voltage to the diac continues for a long period of time, there is the possibility that the diac may be damaged, resulting in loss of the protection of the same. Further, a detection circuit which uses a photo-coupler can merely detect an abnormal voltage across a winding of a balance coil, but no measures after detection of an abnormal voltage is taken.
It is desirable to provide a discharge tube lighting apparatus, a light source apparatus, and a display apparatus to surely solve the above-mentioned problems by low cost.